1. Filed of the Invention
The present invention relates generally to an objective lens driving device installed on an optical head which reads/writes signals to and from a signal recording medium, such as an optical disk, and more specifically to an objective lens driving device having a cover over the top surface thereof.
2. Description of the Related Art
A typical objective lens driving device includes a lens holder to which an objective lens is mounted. The lens holder is connected to a frame by an elastic supporting member so that the lens holder is elastically supported by the frame.
Suspension wires are often used as the elastic supporting member. In an objective lens driving device using suspension wires, one end of each of multiple (typically four) suspension wires is fixed to the lens holder, while the other end of each suspension wire is fixed to a base plate attached to the frame to thereby support the lens holder on the frame movably and elastically.
The objective lens driving device usually has a cover over the top surface thereof to protect the elastic supporting members and a magnetic circuit during assembly or services.
The cover is made of a metal, typically steel, plate configured such that the top surface of a back yoke attached to a magnet can be magnetically attached to the top surface of a facing yoke arranged opposite the magnet. Thus, the cover reduces leakage of magnet flux and raises a magnetic flux density of an air gap formed between the magnet and the facing yoke where an effective edge of a driving coil is located.
A tracking coil is typically placed in the air gap facing one magnetic pole side of the magnet in the lens holder. When the magnetic circuit part of the objective lens driving device is covered by the cover, the height of the tracking coil may be limited by the cover, which may make it impossible to provide a tracking coil of effective length.
Further, if the cover is not securely affixed to the top surface of the back yoke and the facing yoke over the junction of two yokes, as is not uncommon, the increase of the magnetic flux density of the air gap may prove insufficient or contact of the cover against the top surface of the yokes between right and left parts of the cover may prove insufficient, leading to an unbalanced driving force of the lens holder.
In addition, when the lens holder is displaced beyond a normal range in a direction of an optical axis of the objective lens due to abnormal conditions, impact, etc. during installation of a focus servo, the lens holder may impinge on the cover and, if the lens holder is tilted to a great degree, the suspension wires, if used as the elastic supporting members, may be bent.